1. Field of the Invention
The present invention relates to a semiconductor layer structure having distributed strain and an optical semiconductor device including this semiconductor layer structure.
2. Description of Related Background Art
Conventional strained lattice or superlattice structures include a plurality of kinds of semiconductor layers having different lattice constants layered with internal stresses left in the layers. Techniques for utilizing the strained lattice in an optical device, such as a semiconductor laser or a semiconductor optical amplifier, have been proposed, and using those techniques improves performance of various devices.
However, when introducing strain into the semiconductor layer to achieve the strained lattice, it is necessary to avoid introducing lattice defects. For this reason, the thickness, up to which the strained lattice can be constructed, is limited. This is called a critical thickness. The critical thickness has been theoretically and experimentally studied by, for example, Matthew et al. "J. Crystal Growth", 27 (1974) p. 118, and Fox et al. "J. Crystal Growth", 109 (1991) p. 252.
If the strained lattice is layered above the critical thickness, the strain begins to be partially relaxed owing to the occurrence of lattice defects. If the layer thickness is further increased, the strain will become completely relaxed. As a result, the lattice constant proper to the layered semiconductor layer appears and the strain will disappear.
Therefore, in a technical field wherein strain is a desired property, a semiconductor layer with strain therein typically must have a maximum thickness below the critical thickness. Furthermore, the critical thickness decreases as the lattice mismatching between layers increases resulting in a greater amount of strain. Therefore, use of the strained lattice is limited to relatively thin layers and it is generally used in combination with a quantum well structure. A strained lattice may be employed as a well layer, a barrier layer or a multi-layer in which those well layers and barrier layers are alternated.
As discussed above, a thick layer has rarely been constructed with strain existing therein, and use of such a layer has not been contemplated.